The present invention relates to a servo system for controlling the reading position in a data reading apparatus.
In a data reading apparatus such as an optical video disc player, a so-called focus servo system is employed to accurately focus the laser beam on a recording medium, namely, a recording disc. In addition, a so-called "tracking servo system" is used to cause the laser beam focused on the recording surface of the recording disc to accurately follow the track on the recording surface.
An example of a conventional servo system for controlling a reading position employing the above-described focus servo system and tracking servo system is shown in FIGS. 1A and 1B. In FIG. 1, three spot beams 2, 3 and 4, which are obtained by focusing a laser beam, are applied to a recording track 1, the remaining spot beams 3 and 4 are on opposite edges of the track 1. Accordingly, when the detecting spot beam 2 is displaced in a direction perpendicular to the track (radially of the video disc), the difference in light quantity between the reflected beams of the spot beams 3 and 4 changes according to the direction of displacement and the amount of displacement of the detecting spot beam 2. The reflected beams of the spot beams 3 and 4 are converted to electrical signals by photoelectric conversion elements 5 and 6. These electrical signals are outputted as tracking error forming signals A and B, respectively.
The reflected beam of the detecting spot beam 2 is applied through a cylindrical lens (not shown) to the light-detecting surface of a photoelectric conversion element 7. The plane including the generating line of the cylindrical lens is different from the plane perpendicular thereto in the position on the optial axis where the light beam passed through the cylindrical lens is focused. Therefore, the shape of the light beam projected onto the light-detecting surface of the photoelectric conversion element 7 changes according to the positional relationship between the recording surface of the recording disc and the focusing point of the laser beam. In order to detect the change in shape of the light beam projected onto the light-detecting surface of the photoelectric conversion element 7, the element 7 is provided with four light-detecting units which are independent of one another and which are arranged as if divided into four parts by two straight lines. The light-detecting surface of the photoelectric conversion element 7 is arranged at the position where, when the focusing point of the laser beam is positioned on the recording surface of the recording disc accurately, the reflected beam passed through the cylindrical lens becomes substantially circular. Thus the difference in level between the outputs of adders 8 and 9, each adding the outputs of the opposed units of the photoelectric conversion element 7, is indicative of the direction and amount of displacement of the focusing point of the laser beam. The outputs of the adders 8 and 9 are provided as focus error forming signals C and D, respectively. This conventional focus servo device is described in more detail in Japanese Laid-Open Patent Application No. 926/1980.
The outputs of the adders 8 and 9 are applied to an adder 10, the output of which is provided as a read data signal, which is an RF (radio frequency) signal.
A pair of signals, such as the tracking error forming signals A and B or the focus error forming signals C and D, which form an error signal, are applied through resistors R.sub.1 and R.sub.2 to the inverting input terminal and the noninverting input terminal, respectively, of an operational amplifier OP.sub.1. The noninverting input terminal of the operational amplifier OP.sub.1 is grounded through a resistor R.sub.3. The inverting input terminal of the amplifier OP.sub.1 is connected through a resistor R.sub.4 to the output terminal. The operational amplifier OP.sub.1 and the resistors R.sub.1 through R.sub.4 form a differential amplifier circuit 11. A signal corresponding to the difference in level between the pair of signals is amplified by the differential amplifier circuit 11, and is outputted as a tracking error signal or a focus error signal. An offset voltage is applied through a resistor R.sub.5 to the inverting input terminal of the operational amplifier OP.sub.1 to eliminate the offset which may be present in the tracking error signal or the focus error signal due to variations in characteristics between the photoelectric conversion elements 5 and 6, or between the light-detecting units of the photoelectric conversion element 7. The offset voltage is provided at the movable contact of a variable resistor VR.sub.1, and fixed contacts of which are connected between the positive and negative terminals of the power source. The movable contact of the variable resistor VR.sub.1 is grounded through a capacitor C.sub.1.
The output error signal of the differential amplifier circuit 11 is applied through a resistor R.sub.6 to the inverting input terminal of an operational amplifier OP.sub.2, the noninverting input terminal of which is grounded through a resistor R.sub.7. The output of the operational amplifier OP.sub.2 is applied to one end of a coil L through a buffer amplifier 12 composed of transistors Q.sub.1 and Q.sub.2, a capacitor C.sub.2 and resistors R.sub.8 and R.sub.9. The coil L is either a drive coil serving as a tracking actuator for driving a tracking mirror (not shown) or a focus motor rotor serving as a focus actuator for driving a focusing lens (not shown). The other end of the coil L is grounded through a resistor R.sub.10. A voltage corresponding to a drive current flowing in the coil L is developed across the resistor R.sub.10. The voltage thus developed is applied through a feedback resistor R.sub.11 to the inverting input terminal of the operational amplifier OP.sub.2.
In the conventional servo system thus constructed, the tracking mirror or the focusing lens is operated so that the error signal level becomes zero, as a result of which the position of the focusing point of the laser beam on the recording track or on the recording surface is controlled accurately.
The above-described conventional servo system is, however, disadvantageous in that when the difference in level between the pair of signals forming the error signal is eliminated, such as may be effected by scratches, dust or smudges on the recording medium, only the offset voltage is amplified, thus supplying an unwanted drive current to the actuator. As a result, erroneous operations such as defocusing and track jumping may occur.
Accordingly, an object of invention is to provide a servo system for a data reading apparatus which can control the reading position without being affected by scratches, dust or smudges on the recording medium.